The clock replaces NIST-7, a cesium beam atomic clock used from 1993 to 1999. NIST-F1 is ten times more accurate than NIST-7. NIST-F1 will be replaced by the NIST-F2.

Frequency measurement

The apparatus consists of counter-propagating lasers that cool and trap a gas of cesium atoms. Once trapped, two vertical lasers propel the atoms upward inside a microwave chamber. Depending on the exact frequency of the microwaves, the cesium atoms will reach an excited state. Upon passing through a laser beam, the atoms will fluoresce (emit photons). The microwave frequency which produces maximum fluorescence is used to define the second.